S. Aime et al.
which are parahydrogenated with heterogeneous cata- (20 mL) was added to a solution of sulfuric acid (96%, 0.45 mL)
SHORT COMMUNICATION
[
31]
in methanol (10 mL). The mixture was stirred for four days at room
temp. Then water (25 mL) was added, and the ester was extracted
with diethyl ether (3ϫ20 mL). The organic solution was washed
with water, a saturated aqueous solution of sodium carbonate and
water again; then it was dried with sodium sulfate. The product
lysts.
Obviously, the spin relaxations induced in systems
with long molecular correlation times (here represented by
the immobilised catalysts) are largely diminished by the
short lifetime of the H2 molecules at the metal centres.
These considerations may be relevant for the exploitation
of PHIP effects in heterogeneous catalysis.[
was filtered, and the solvent was evaporated under vacuum. The
31]
1
yield was 67%. H NMR (400 MHz, [D
6
]acetone): δ = 1.99 (s, 3
]acetone): δ =
53.38 (s), 85.02 (s), 71.73 (s), 51.56 (q), 1.99 (q) ppm.
1
3
H), 3.70 (s, 3 H) ppm. C NMR (150 MHz, [D
6
1
Conclusions
Synthesis of Methyl cis-2-Butenoate: Inside a three-necked flask
equipped with valves for connection to a vacuum pump, a manom-
eter and a hydrogen reservoir, sodium 2-butynoate (1.16 g) was dis-
The detection of the PHIP effect in conjunction with ki-
netic measurements allowed to get more insight into the
mechanism of olefin isomerisation during catalytic alkyne
hydrogenation. It was found that hydrogen coordination is
necessary but no H-scrambling takes place between the co-
solved in methanol (15 mL). A mixture of Pd/BaSO
oline (20 mg) and water (3 µL ) was added, the flask was frozen in
liquid N and degassed, then it was warmed to room temp., and
4
(20 mg), quin-
2
hydrogen (1.2 atm) was introduced. The reaction mixture changed
colour from brown to black, indicating that hydrogenation had
started. The mixture was stirred up to the end of hydrogen con-
ordinated H molecule and the H atoms on the coordinated
2
olefin. The isomerisation step involves an intermediate con-
taining one solvent and one hydrogen molecule bound to sumption (about 2 h). Then it was filtered, and the solvent was
the Rh centre, whereas the organic fragment is freely rotat- removed under vacuum. The residue was dissolved in water
ing at the Rh–C σ-bond.
(10 mL), HCl (37%, 0.8 mL) was added, and the obtained cis-2-
butenoic acid was extracted with diethyl ether (4ϫ7 mL). The ether
solution was desiccated with sodium sulfate, filtered and dried un-
der vacuum. cis-2-Butenoic acid was then dissolved in methanol
Experimental Section
(
20 mL), and sulfuric acid (96%) (0.2 mL) was added. The solution
1
H NMR PHIP spectra were recorded with a Bruker Avance-600
was stirred at room temp. for four days. Then water (20 mL) was
added, and the ester was extracted with diethyl ether (3ϫ20 mL).
The organic solution was washed with water, a saturated aqueous
solution of sodium carbonate and water again; then it was dried
1
spectrometer operating at 600 MHz. H NMR spectra for kinetic
evaluations were recorded with an EX-400 JEOL spectrometer op-
erating at 400 MHz.
with sodium sulfate. The product was filtered, and the solvent was
Para-enriched hydrogen (50%) was prepared by storing H
2
with
1
evaporated under vacuum. The yield was 60%.
400 MHz, [D
H NMR
Fe at 77 K for one hour.
2
O
3
(
6
]acetone): δ = 2.09 (d, 3 H), 3.65 (s, 3 H), 5.77 (dd,
H); 6.38 (m, 1 H) ppm. 13C NMR (150 MHz, [D
Parahydrogenation reactions were carried out in a 5-mm NMR
tube equipped with Young valve. The catalyst
Rh(COD)(dppb)][BF ] (10 m in [D ]acetone) was activated by re-
1
1
6
]acetone): δ =
67.14 (s), 146.6 (d), 121.2 (d), 51.2 (q), 15.0 (q) ppm.
a
[
4
6
action with normal hydrogen, the tube was then opened, and the
substrate methyl 2-butynoate (1) (100 m) was quickly added. The
tube was frozen, and parahydrogen (1.2 atm) was introduced (cor-
responding to 3.8 atm at room temp.); the sample was then warmed
to room temp., shaken for ten seconds and introduced into the
spectrometer. Single-scan H NMR spectra (with 45° pulses) were
recorded at different times (from 20 to 120 s) after the introduction
of the sample in the spectrometer.
[
[
1] R. Eisenberg, Acc. Chem. Res. 1991, 24, 110–116.
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Concepts Magn. Reson. Part A 2006, 28, 321–330.
[
3] S. B. Duckett, C. L. Newell, R. Eisenberg, J. Am. Chem. Soc.
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1
[
4] S. B. Duckett, G. K. Barlow, M. G. Partridge, B. A. Messerie,
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[5] J. Bargon, J. Kandels, P. J. Kating, J. Chem. Phys. 1993, 98,
6150–6153.
Kinetic measurements were carried out as follows: Inside a Schlenk
[
6] J. Bargon, J. Kandels, P. J. Kating, A. Thomas, K. Woelk, Tet-
rahedron Lett. 1990, 31, 5721–5724.
tube, equipped with a rubber septum, [Rh(COD)(dppb)][BF
15 mg) was suspended in deuterated solvent (CD OD or [D
acetone, 2.5 mL), this suspension was frozen and degassed, and H
1 atm) was introduced into the tube. The resulting mixture was
4
]
(
3
6
]-
[
7] C. R. Bowers, D. P. Weitekamp, Phys. Rev. Lett. 1986, 57,
2
2645–2648.
(
[8] C. R. Bowers, D. H. Jones, N. D. Kurur, J. A. Labinger, M. G.
stirred at room temp. until complete dissolution of the catalyst: this
indicated the activation of the catalyst by hydrogenation of the
COD ligand. The clear orange solution was then frozen again, and
H was replaced with a 1:1 mixture of H and Ar, or with Ar alone,
2 2
depending on the type of measurement. The tube was warmed up
Pravica, D. P. Weitekamp, Adv. Magn. Reson. 1990, 14, 269–
291.
[
[
[
9] S. Aime, R. Gobetto, F. Reineri, D. Canet, J. Chem. Phys. 2003,
119, 8890–8896.
10] R. Gobetto, L. Milone, F. Reineri, L. Salassa, A. Viale, E. Ro-
senberg, Organometallics 2002, 21, 1919–1924.
11] J. Natterer, J. Bargon, Prog. Nucl. Magn. Reson. Spectrosc.
to room temp., then a solution of methyl cis-2-butenoate (22 mg)
in CD OD or [D ]acetone (0.5 mL) was added by a syringe. Ali-
3 6
1997, 31, 293–315 and references cited therein.
quots of 0.25 mL were taken at successive times and were analysed
[12] S. B. Duckett, C. J. Sleigh, Prog. Nucl. Magn. Reson. Spectrosc.
1999, 34, 71–92 and references cited therein.
1
by H NMR spectroscopy. Concentrations of the cis and trans iso-
mers were determined by integration of the spectra, and were re-
ported as a function of time for the evaluation of the kinetic con-
stants.
[13] S. Aime, R. Gobetto, D. Canet, J. Am. Chem. Soc. 1998, 120,
6770–6773.
[
14] S. Aime, W. Dastrù, R. Gobetto, A. Viale, D. Canet, J. Phys.
Chem. A 1999, 103, 9702–9705.
[15] S. Aime, D. Canet, W. Dastrù, F. Reineri, A. Viale, J. Phys.
Chem. A 2001, 105, 6305–6310.
Synthesis of 1: Compound 1 was prepared by esterification of 2-
butynoic acid: 2-butynoic acid (0.9 g, 10.6 mmol) in methanol
4350
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